Ultrasonic Power Transfer for Ultra-High-Frequency Biphasic Electrical Neural Stimulation

Abstract

Neurostimulators have been developed over the past few decades to treat various diseases, such as Parkinson’s disease, chronic pain, epilepsy, migraine and bladder dysfunction. One of the major design challenges is the selection of a powering method that could supply mW power levels to miniaturized implanted devices. Among several methods, wireless power transfer is often used to directly supply the electronics or to recharge an implanted battery. In particular, the interest in ultrasonic waves as source of power and data for implantable medical devices has recently grown, particularly due to the advantages over RF and inductive coupling power transfer. In fact, ultrasounds can deliver higher power to mm-sized and deeply (> 10 cm) implanted receivers than the other techniques. Moreover, acoustic waves do not interfere with electromagnetic fields.Conventionally, biphasic constant current or constant voltage pulses are selected for stimulating nerve tissue. The first (usually cathodic) phase has the purpose of activating the excitable nerve fibres, while the second (usually anodic) phase reverses the direction of the stimulation current to avoid long-term accumulation of charge. Alternatively, Ultra-High-Frequency stimulation can be employed. This consists of current or voltage pulses at a high frequency (≥ 1 MHz), which are obtained from a DC source and the operation of high-frequency switches. In both techniques, when a wireless supply is chosen, the received AC signal is rectified, stored and regulated to operate the usually low-voltage blocks of the rest of the system. Nonetheless, up-conversion of the signal is often required to provide enough voltage compliance to operate an output stage connected to the stimulating electrodes.Taking a more radical approach, it is possible to avoid the lossy conversion from AC to regulated DC and from that to high-frequency stimulation by eliminating the need of power storage, regulation and up-conversion. To this end, the aim of this work was to design a circuit topology that generates ultra-high-frequency pulses by rectifying the sinusoid obtained from an ultrasound transducer and using the obtained waveform to directly stimulate the tissue in a biphasic fashion. This could result in a highly efficient and miniature circuit, which has the potential to be used for stimulating small peripheral nerves. Simulations in LTSpice were conducted to analyse the performance of the proposed system. The operation was then verified by measurements with a fabricated prototype, which was capable of providing biphasic pulses by receiving a signal from ultrasound piezoelectric transducers and driving a pair of electrodes.